For the bumpers of motorcars, a very high impact resistance is required and thermoplastic resin compositions, for example, polyolefin resin compositions mainly comprising a propylene polymer, an ethylene-propylene copolymer rubber and talc, are widely used and also various resin compositions are proposed as described, e.g., in JP-A-53-64256, JP-A-53-64257, JP-A-57-55952, JP-A-57-159841, and JP-A-58-111846 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Many of the bumpers for motorcars are coated products for increasing the designs thereof in addition to satisfy the performance.
Recently, in particular, the environmental issue, the conservation of resources, etc., have been regarded as important and thus the re-using (recycling) of various products, particularly, motorcar parts, has been required.
As a recycling method of the bumpers of motorcars composed of coated thermoplastic resin compositions, a method of crushing the recovered resin moldings by a crusher and utilizing the crushed moldings as raw materials for moldings or a method of after pelletizing the crushed moldings by an extruding machine, and utilizing the pellets as the raw materials for moldings is generally used.
However, when waste coated bumpers are used as recycling materials by these method described above, since the coated resins are poor in the compatibility with a thermoplastic resin such as a polyolefin resin, there is a problem that the impact resistance of the moldings composed of the resin composition prepared by re-using the waste bumpers is lowered. Accordingly, it is difficult to use the re-used materials as the parts requiring the impact resistance as they are.
As a method of improving the impact resistance of the moldings composed of the resin compositions utilizing the waste motorcar parts composed of coated polyolefin resins, the methods described in JP-A-5-154861 and JP-A-5-169479 are known.
In the methods described above, at least 76 parts by weight of various materials compounded for improving the impact resistance are required and thus, the amount of the waste materials becomes relatively small and the use of such a large amount of the compounding materials increases the production cost. Accordingly, it is difficult to completely recycle waste materials existing and generated in a large amount, and thus the foregoing methods are not so effective as a recycling method of waste materials.
In the field of injection molding of thermoplastic resins, sandwich molding for obtaining multilayer moldings has hitherto been practiced. In this method, while the material used for a core material may be same as the material used for a surface layer, a multilayer molding can be produced by using different materials for the core material and the surface layer, whereby a molding having characteristics that are not obtained by using the same kind of material for both the core material and the surface layer is obtained. Because of these advantages, this molding method has recently been widely used in the field of household electric appliances, the field of motorcars, etc., for improving the appearance (prevention of shrinking, bending, etc.) and for the improvement of the rigidity.
In order to avoid the foregoing defects of the recycling materials, a method of restraining lowering of the properties by using the recycling material for the core material and using a new polypropylene resin for the surface layer in sandwich molding has been employed as described, e.g., in JP-A-57-11052, JP-A-61-259943, JP-A-62-28332, JP-A-63-237924, and JP-A-6-71829.
However, even the sandwich molded product as described above shows severe lowering of the properties as compared with the molded product composed of a new resin alone, and the foregoing method cannot thus be applied for the product required to have a high impact resistance at low temperature, such as, exterior parts of motorcars, for example, bumpers, under the existing circumstances.
Since polypropylene does not have a polar group, the coating property onto the polymer moldings of polypropylene is poor, and it has generally been practiced to surface treat polypropylene moldings with the vapor of an organic chlorine solvent such as trichloroethane (TCE), etc., for improving the coating property and then, after coating thereon a primer for hiding the color of the molding itself, to coat the molding with a coating material.
However, since a global environmental issue has been highlighted as a world-wide tendency, it will become impossible to use TCE which is generally used for the pre-treatment of coating.
Also, under the tendency of resource saving and cost saving, a so-called gasohol, i.e., a mixed fuel obtained by mixing gasoline with lower alcohols (methanol, ethanol, butanol, etc.) tends to be used as the fuel for motorcars, etc. Since the gasohol adversely affects the adhesion of a base material to be coated and a polyurethane coated layer, there is a possibility that the coated layer is released with the action of the gasohol.
Thus, for solving such a problem, a composition composed of a propylene polymer, an OH group-containing compound, a thermoplastic elastomer, an oligomer having a functional group at the terminal, etc., is proposed as described, e.g., in JP-A-7-26044.
However, although the gasohol resistance can be improved by using the foregoing composition as the surface layer in the sandwich molded product as described above, the properties thereof are greatly lowered as compared with the molded product of a new resin alone, and hence the foregoing composition cannot be applied to the products requiring a high impact resistance at low temperature as exterior parts of motorcars, such as bumpers.
In ordinary sandwich moldings, the kind of resin materials can be changed between the core material and the surface layer, but in the sandwich moldings formed in JP-A-7-26044, there are problems that the low-temperature impact resistance is insufficient and the moldability is inferior for large-sized products.
In the moldings described in JP-A-4-191044, the coating property is not improved and there may be a problem in coating the composition without applying the trichloroethane treatment.